In current communication networks, the users require increasingly higher data transmission rates. Transmission methods which can provide high transmission rates also in the subscriber access networks of a communication network are, for example, methods which operate in accordance with the xDSL method. Using these transmission methods, the operators of communication networks can also offer their customers broadband connections, for example to the Internet, by means of which the subscribers can utilize numerous applications in an increasingly simpler and more rapid manner. An example of such a data-intensive application for which high transmission rates are needed is, for instance, the transmission of real-time-related multimedia files.
In particular, for example, popular television channels will also in future be increasingly transmitted via packet-oriented communication networks such as, for example, an IP (Internet Protocol) network. This service is considered to be part of the so-called “triple plays”, which means that three—originally separate—data services (telephone, Internet, television) are conveyed via the same communication network.
In this context, the information is usually transmitted in so-called streams in the case of television transmissions via an IP network (IP TV). This means that a central server converts the television channels into a continuous flow of data (the channels are “streamed”) and these data streams are distributed to individual subscribers. As part of such a distribution service, the data stream is received by the individual subscribers, converted back and subsequently displayed on a screen.
To be able to successfully offer this type of television transmission, however, it is necessary to observe that the subscribers or the customers are also provided with at least the same quality of television pictures as is the case with current TV networks (cable, satellite, aerial) with such a streaming method.
This specifically means that, on the one hand, adequate picture quality is achieved, for example. Especially in the case of video transmissions, transmission errors become apparent in errors in the picture representation which are easily recognized and therefore extremely disturbing. Thus, for example, the picture may appear blurred or so-called block errors occur, i.e. individual parts or blocks of the picture are represented wrongly.
On the other hand, however, to successfully offer IP TV, it must also be ensured that further features or characteristics to which the subscribers to conventional television transmission networks are used are also offered in the case of IP TV. One of these characteristics is a shortest possible switch-over time when switching between two television channels.
While this is normal with conventional TV networks since all receivable channels are simultaneously present at the connection of the subscriber in these networks, it is different in the case of IP-based (or Ethernet) networks: in such networks, only the requested channels are ever present at the connection of a subscriber. In principle, a subscriber in an IP network informs a central server that he wishes to receive a certain stream (a certain television channel), i.e. the subscriber makes a request for a communication connection with this server. The server thereupon forwards the corresponding stream to the subscriber. In the case of a switch-over process, which corresponds to a change of the stream, this switch-over must therefore first be notified to the server by means of various protocols and finally be carried out by the server (or by two servers, if both streams come from different servers). With each switch-over of the television channel, a new stream must thus be requested and set up (or a new communication connection established), a method which is more elaborate and therefore needs more time than the switch-over in conventional TV networks.
In current IP networks, the distributing of a stream to one or more subscribers is achieved by means of so-called multipoint connections. In this context, a data stream can be conveyed from a starting point or transmitter (for example a central server within the IP network) simultaneously to several receivers or subscribers. This is a “point-to-multipoint” connection or a so-called “multicast” method or multicast transmission.
Multicast transmissions are currently controlled in most cases by means of the “Internet Group Management Protocol” (IGMP). The IGMP is based on the Internet Protocol and provides for group communication. In a multicast transmission (multicasting), single IP packets are simultaneously distributed under an IP address to several stations (to a group of stations). To reduce the volume of data produced, if possible, the IGMP provides the possibility of distributing groups dynamically. Individual subscribers or connections are thus allocated to certain groups with a particular offer. However, these groups are not administered in the transmitting station but, for example, in individual network nodes or in the routers along the way to the receiver. For this purpose, information about the outgoing interface at which a station or a connection is located to which certain multicast IP packets are to be forwarded is stored in the routers. In the IGMP, functions are implemented by means of which, for example, the routers can communicate with one another and by means of which stations can inform a router that the corresponding subscriber wishes to receive certain multicast IP packets.
In multipoint connections managed in this manner, it is thus in most cases not known in the transmitter which or even how many subscribers receive its IP packets. According to IGMP, the transmitter only sends out a single data packet to the corresponding higher-level router. It is only within the individual routers of the communication network that the IP packets are finally multiplied if necessary. This is done in dependence on the number of stations connected to the respective routers which are intended to receive the corresponding IP packets. Thus, all network nodes involved in the communication must support a multicast protocol such as IGMP.
In multicast protocols, commands for requesting IP packets also exist in addition to numerous other protocol commands: for example, a subscriber applies for membership in a group by means of the so-called “join” command (“JoinGroup”), i.e. the subscriber may request a certain stream or television channel whereupon he becomes a further receiver of the corresponding IP packets.
If a subscriber wishes to leave a group of which he is currently a member, this can be done in accordance with various methods according to the prior art. On the one hand, this can be done under timer control (e.g. IGMP Version 1), on the other hand, a membership can also be terminated directly by means of a suitable command (e.g. IGMP, Version 2). In the case of timer-controlled methods (like the IGMPv1 in which there is no terminating command), according to the protocol a subscriber device sends to the corresponding higher-level node, at predetermined time intervals, information which indicates that the respective subscriber still wishes to remain a member of said group. In the case where no such information is received at the network node within the predetermined time interval (when the timer in the network node has expired), the connection is terminated by the network node.
In other multicast protocols, there are certain commands for deliberately terminating a membership (e.g. IGMPv2: “LeaveGroup”) Although the possibility thus exists in such cases to deliberately leave certain groups, numerous current multicast implementations such as, among others, for instance current set top boxes, still continue to operate without this “leave” command. In such cases, too, leaving a group is then controlled by timer.
A further possibility for ending membership of a group is the so-called “fast leave”. This is an implicit ending of membership. This means that with each respective new “join” request by a subscriber, that is to say whenever the subscriber, for example, applies for a new television channel, the previous membership of another group is automatically ended. However, this method can only be used in rare cases since a prerequisite for this is that the subscriber in each case only has or wishes to have a single data stream.
Management of the individual subscribers and their group membership, that is to say management of the multicast connections can also be carried out, for example, centrally. Information regarding which connected subscribers have subscribed to which IP data streams or belong to which groups can thus be stored, e.g. in a central management unit or also in the DSLAM itself. For this purpose, for example, a table can be set up in which the currently subscribed IP streams are stored for each subscriber connection or else for each “media access control” address (MAC address), i.e., even in the case where several terminals are connected to one subscriber line, the group membership of each individual terminal is logged by means of the MAC address which is unambiguous for each device connected to an Ethernet. The information about the group membership can be recorded directly in the DSLAM, for example by monitoring the IGMP (“IGMP snooping”).
In most cases, the following disadvantage is produced when leaving a group, or when subscribing to a new group, particularly in the abovementioned IP TV applications: if a subscriber subscribes to a new group, groups previously subscribed to are not always automatically left. This leads to the new applications in most cases not being able to immediately access the required bandwidth since a part of the existing bandwidth is still reserved for the previous application no longer required. Especially in the case of, for example, IP TV, this leads to an extension of the abovementioned problem of longer switch-over times since the newly selected channel cannot be displayed unless the transmission resources required for displaying the previous channel have first been released again.